The present invention relates to a plant for transmitting electric power comprising a direct voltage network for High Voltage Direct Current and at least one three-phase alternating voltage network connected thereto through a station, in which the station is configured to perform transmitting of electric power between the direct voltage network and the alternating voltage network and comprises at least one Voltage Source Converter configured to convert direct voltage into alternating voltage and conversely and a unit configured to control said converter according to a Pulse Width Modulation (PWM) pattern for generating an alternating voltage in each of three phase legs of the converter having a third harmonic voltage part added to a fundamental voltage, where the fundamental voltage part has the same frequency as the alternating voltage on said alternating voltage network.
Plants of this type are for example known from the brochure “It's time to connect, Technical description of HVDC Light® technology”, issued by ABB Power Technologies AB, Ludvika, Sweden, printed by Elanders, Västerås POW-0038 rev. 2, 2006 February.
One type of such known Pulse Width Modulation, which only adds a third harmonic voltage part to a fundamental voltage part, will be discussed below with reference made to FIGS. 1-4 for forming a base to illuminate the invention but not in any way restricting the scope thereto. The control unit of this type may utilize other types of Pulse Width Modulation adding other zero sequence voltage parts to the fundamental voltage part, such as sixth and ninth harmonic voltage parts and these are also comprised.
Furthermore, said Voltage Source Converter may be of any known type, such as two-level, three-level, multi-level Voltage Source Converter and also of the so-called Modular Multi Level Converter-type of M2LC. The frequency used for the pulses of said Pulse Width Modulation pattern is dependent upon which type of Voltage Source Converter is used, so that this frequency is typically in the order of 1 to 5 kHz for a two-level converter and 100 Hz to 500 Hz for a M2LC-converter, in which the frequency of the alternating voltage on said alternating network is typically 50 Hz or 60 Hz.
A two-level Voltage Source Converter is very schematically shown in FIG. 1. The converter 1 has three phase legs 2-4 connected between opposite poles 5, 6 of a direct voltage network 7. Each phase leg has two current valves 8-13 connected in series and a midpoint therebetween forming a phase output 14-16 and being connected through phase reactors 17 and a transformer 18 to an alternating voltage network 19.
A unit 20 is configured to control semiconductor devices 21 of turn-off type, such as IGBTs, to turn on or turn off for connecting the respective output to the potential of the pole 5 or the pole 6 and by that generating voltage pulses according to a Pulse Width Modulation pattern on the respective phase output. How this is done is conventional technique known to those with skill in the art. The phase reactors 17 will help to smooth out the alternating voltage so created.
FIG. 2 is a diagram of voltage U versus time t showing for one phase the voltage A to the left of the corresponding phase reactor in FIG. 1 when Pulse Width Modulation is carried out without adding any harmonic voltage part to a fundamental voltage part, whereas B illustrates a third harmonic voltage part that may be added to the fundamental voltage part and C illustrates the voltage obtained when adding a third harmonic voltage part to a fundamental voltage part, namely by adding the curves A and B. It appears that the third harmonic voltage part decreases the peak voltage of the converter. This means that the fundamental voltage part may be increased by as much as 15 percent with respect to the case of no addition of the third harmonic voltage part and the voltage obtained by such an addition will still remain under the limit of the converter voltage. This means that up to 15 percent rated power may be gained and the converter losses may also be reduced by around 15 percent. This is the reason for adding such a third harmonic voltage part during said Pulse Width Modulation. FIG. 3 illustrates the appearance of the converter alternating voltage U for the three phases 2-4 versus time for such a Pulse Width Modulation having a third harmonic voltage part added to a fundamental voltage part for each phase.
FIG. 4 shows the appearance of the phase voltages of FIG. 3 on the alternating voltage network 19 after having passed the transformer 19. It appears that the third harmonic voltage parts are cancelled on the alternating voltage network side of the transformer resulting in sinusoidal alternating phase voltages aimed at and having a higher peak voltage value than possible to obtain without the addition of said third harmonic voltage part.